As part of the continuing effort to reduce the environmental impact of various industrial chemical processes, there has been a strong emphasis in developing new methodology for the application and cure of organic coatings. While these ubiquitous materials are absolutely essential to modern life, they also constitute one of the primary industrial sources of emissions of volatile organic solvents that contribute to air and water pollution. The use of photopolymerizations for the fabrication of decorative and protective organic coatings is one solution to this problem that is receiving increasing acceptance as this technology matures and as more users gain experience in applying it to their specific requirements.
Two main types of monomer compositions are commonly used in photopolymerization processes, acrylate and epoxy. It is widely recognized that the cationic photopolyinerization of epoxy compositions is slower than free radical photopolymerization of acrylate monomers. For this reason, although photocured epoxies generally have better properties than their acrylate counterparts, these materials are not commonly used for high speed applications such as printing inks and rapid imaging techniques.
One method for increasing or accelerating the polymerization rate of epoxy monomers in cationically photoinitiated processes is through the use of photosensitizers. In fact, photosensitizers are critical to the success of cationic photopolymerizations in many applications in which photopolymerizations are employed. When broad band emitting light sources are used, the additional spectral sensitivity provided by a photosensitizer often permits the capture of a higher fraction of the available light emitted from most common UV irradiation sources. As a result, more efficient photolysis of the photoinitiator takes place generating a larger number of initiating species that produces an apparent acceleration of the rate of polymerization of the monomer as compared to when the photosensitizer is absent. In addition, there is currently a tendency toward the use of monochromatic light sources such as lasers and light emitting diodes for imaging applications; use of photosensitizers may be necessary when these light sources emit at wavelengths not absorbed by the photoinitiator.
Polycyclic or polynuclear aromatic compounds are known photosensitizers for photolysis of onium salts. These compounds are readily available and, in many cases, inexpensive starting materials. Further, they generally have very rich and strongly absorbing UV and visible absorption spectra with the potential for sensitization in the very important long wavelength UV and the visible regions. Monomeric and polymeric compounds containing the carbazole nucleus have been described by Chen, et al (Chen, Y.; Yamamura, T.; Igarashi, K. J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 90) and by the inventor of the present subject matter (Hua, Y.; Crivello, J. V. J. Polym. Sci. Part A. Polym. Chem. 2000, 38, 3697; Hua, Y.; Crivello, J. V., Macromolecules 2001, 34, 2488). Phenothiazine derivatives have also been reported as a new class of photosensitizers that can be employed for onium salt photoinitiators (Gomurashvili, Z.; Crivello, J. V. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 1187; Rodriguez, M. R.; Neumann, M. G. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 46; Crivello, J. V.; Lee, J. L. Macromolecules 1983, 16, 864; Denizligil, S.; Resul, R.; Yagci, Y.; McArdle, C.; Fouassier, J.-P. Macromol. Chem. Phys. 1996, 197, 1233). Among the most efficient photosensitizers for onium salts that have been discovered are electron-rich polynuclear aromatic compounds such as anthracene, pyrene and perylene (dibenz [DE, KL] anthracene) (Crivello, J. V.; Lam, J. H. W. J. Polym. Sci. Part A: Polym. Chem. 1979, 17, 1059). U.S. Pat. No. 6,313,188, to Takahashi, discloses photocatalytic compositions containing polycyclic aromatic compounds and carbazole derivatives, substituted with a hydroxy group, an optionally-substituted aralkyloxy group or an alkoxy group, including optionally substituted 9,10-dialkoxyanthracenes and 9,10-diaralkyloxyanthracenes, and, specifically, 9,10-dimethoxyanthracene, 9,10-diethoxyanthracene, 9,10-dibenzyloxyanthracene and 2-ethyl-9,10-dimethoxyanthracene. The compositions are reported to cure at an accelerated rate. Yet, despite the many potential applications for this class of photosensitizers, they have received little attention. This results from several major deficits associated with these compounds: First, they tend to be poorly soluble in most monomers and, second, many have high vapor pressures at room temperature and are thus easily lost from thin film coatings during polymerization. Third, phenolic-type photosensitizer compounds, that is polycyclic aromatics substituted with a hydroxy group, such as those described in U.S. Pat. No. 6,313,188, are subject to oxidation and yellow or darken as a result. This can be a problem in clear coating applications, or where a pigment is utilized. Finally, most, if not all, of the polynuclear aromatic hydrocarbons that have been disclosed as photosensitizers for cationic polymerizations are toxic. Toxicity is of concern not only in the context of safety in the workplace where the formulation and polymerization of the compositions is performed, but also in the final application, where toxic compounds that are not bound into the polymer network can be leached out. In particular, for applications where the cured compositions may be in contact with food, it is essential to use photosenzitizers that become bound to the growing chain during the polymerization. It is, therefore, of considerable value to find non-phenolic photosensitizers that may be chemically incorporated into the polymer network and by which the rate of the photopolymerization of epoxy compositions can be conveniently and simply accelerated to put them on par with photocurable acrylates.